Having my Leviathan die on Turn 2, I had more luck using Shell Armor on the first turn with my Hydraling.

TD Script

ability(#3) [ ! self.hp.full & ! self(#3).active ]
ability(Shell Armor:1380) [ enemy.aura(Crouch:164).exists ]
ability(#1) [ self.aura(Toxic Skin:1086).exists ]
ability(#2)
change(next)

This script attempts to deal some damage attack on every round. Passing or reapplying Toxic Skin only increases the chance of losing the fight.

Using these pets instead of passing after using toxic on leviaton go whirlpool and tail slap till he dies, one slap actually hit, when leviathon died and I bought in hydra only one swallow was required to kill foulclaw and had enough health to do a second if necessary.

Your only sure bet to do damage before either of your first two pets die after getting Toxic skin up is with your Gulp Froglet. As long as Foulclaw is below 25% when the Hydraling's last attack before Crouch is active again, you'll likely win.

Wish we could thumbs up a strat when it works (not in the comments. Just in case a new main strat is posted, bumping this to an alt. Gulp Froglet, Leviathan Hatchling, Hydraling - worked 1st time. Just as suggested

Frostshell Pincher 111
Gulp Froglet 122
Ghostshell crab 121

1 - Frostsheel- Rip, Blood in the Water, spiny carapace, dead
2 - Gulp Froglet Toxic Skin, Corpse Explosion, dead
3 - Ghostshell, Bubble, Spirit Claws - Boss should die now, otherwise use Spite claws again.
WIN

tested 3x and worked 100%, Ghostshell with +1000 hp left.

Frostshell would be 112 for this combo but pretty RNG dependent like the other two strats above. #2 above is nearly unworkable. This is better than that one.

Tried Ulathan's strat several times, Gulp Froglet (BB) never made it to the CE turn :(

Very RNG dependent much like other strats. Gulp Froglet died 2/3 times before corpse explosion. Ghostshell Crab would miss and I would lose. This fight is so frustrating.

I couldn't make this config work until I dropped spiny carapace (which has no effect anyway) and switched to Gulp Frog on the 3rd move. Also depended on frog only taking two hits after Toxic Skin.

Thank you Gamara :) that worked like a charm for me (thank god spirit claws hit)
Just for the sake of quick read... TLDR:
Fen Crab/Frostshell Pincher > Rip > Blood in the Water > Gulp Froglet > Toxic Skin > Corpse Explosion > Ghostshell > Bubble > Spirit Claws.

Zangar Crawler - P/P, 1-2-2
Gulp Froglet - B/B, x-2-2
Young Mutant Warturtle - H/P, x-2-2

Zangar Crawler:
1 - Rip
2 - Blood in the Water

If HP of Zangar Crawler is more than 700 then Shell Shield, pass, pass, Crawler dies.
Otherwise swap to Young Mutant Warturtle, Smoke Bomb, swap to Zangar Crawler, Crawler dies. It's important that Crawler dies when Foulclaw's Crouch has 2 turns left, so Gulp Froglet come into battle with only 1 turn of Foulclaw's Crouch left.

Gulp Froglet:
1 - Toxic Skin
2 - Corpse Explosion, if Froglet dies before Corpse Explosion, restart. This turn is RNG based, so H/P and H/B breeds will help. But I have only B/B and probability for me was something like 50/50.

Young Mutant Warturtle:
1 - Tornado Punch
2 - pass
3 - Smoke Bomb
4 - pass, Foulclaw should die here

Took a few tries but it worked! I used a Fen Crab instead of the Zangar Crawler, same abilities. Thanks a lot :D

I think breed for the 1st pet is important. I was failing until I switched to Fen Crab PP breed. Worked great

Tried every strat on here and this was the only one I could get to work. Thanks!

Had to do it twice, but worked great!

P?P on crab counts, I too was failing till I switched. Power counts

Worked first time. Well done. Thank you.

Worked first time. Thanks!

I tried sending this with the "Suggest a Strategy" button but it kept giving me an error, so I thought I'd leave this here. All of the working strategies here depend on the Gulp Froglet, which I don't have, so I made my own strategy:

Pets:
Ghostshell Crab (1/2/1)
Moonshell Crab (-/-/2)
Ghostshell Crab (1/2/1)

Note: Only the Moonshell Crab has different breeds, of which any should work (H/H is best, H/P manages fine, the other two breeds require more luck). Moonshell Crabs are very easy to obtain, so catching a H/H or H/P variant shouldn't take too long.

Important: This fight is highly based on RNG, so even if everything goes well, Foulclaw might still have health remaining after round 13, requiring you to restart the fight.

1. Spirit Spikes (3)
2. Spirit Claws (1) (if this misses, restart the fight)
3. Bubble (2)
4. Swap to the Moonshell Crab
5. Moon Tears (3) (if the crab dies before it can use Moon Tears, restart the fight)
6. Swap to the bubbled Ghostshell Crab
7. Spirit Spikes (3)
8. Spirit Claws (1)

Did the Ghostshell Crab die before it could use Spirit Claws?
Yes:
Switch to Moonshell Crab and keep passing until it dies (the remaining Ghostshell Crab comes in)
No:
Keep passing until it dies, then switch to the remaining Ghostshell Crab

11. If the last Ghostshell Crab is your active pet at this point (Foulclaw's Crouch has two rounds remaining rather than one): Bubble (2)
12. Spirit Spikes (3)
13. Spirit Claws (1)

Even though the win rate isn't ideal, it's a one-time fight, and the pets required are fairly easy to acquire:
-Ghostshell Crabs can be purchased from the fishing vendor during Darkmoon Faire with Darkmoon Daggermaws (obtained by fishing on the Darkmoon Island). Ghostshell Crabs can also be bought from the auction house.
-Moonshell Crabs can be found roaming on the northeastern shores of Draenor Shadowmoon Valley.

This worked great. Thank you.

I used fawn (2/1/2], Rapana whelk (1/1/1), Shimmershell Snail (1/1/1]
Start with stampede, and follow up with headbutt. Your Fawn dies.
Follow up with Rapana whelk. Use Acidic goo, dive, ooze touch until your snail dies.
Shimmershell snail. Use Acidic goo, dive. Foulclaw should be dead.
Note: If your Fawn dies before it can use headbutt, reset the fight

Edit: This is for critters. Sorry.

I didn't have a lvl'd Gulp. I tried so many other idea's and in the end, leveled my gulp and it worked first time.
Clever strat, that gulp makes all the difference in this fight. Could have probably found a sub for the other two, but gulp is key.

perfect! ty

Yep, I applaud whoever came up with this one. It wouldn't have occurred to me that toxic skin could work so well against the enemy's crouch. Well done!

Yeah, percentage base hits like this, trample, and glowing toxin ignore all percentage based reductions like crouch and specific number reductions like stone skin.